Energy-Efficient Ethernet is a set of enhancements to the twisted-pair and backplane Ethernet family of computer networking standards that allow for less power consumption during periods of low data activity. The intention was to reduce power consumption by 50% or more, while retaining full compatibility with existing equipment.[1] The Institute of Electrical and Electronics Engineers (IEEE), through the IEEE 802.3az task force developed the standard. The first study group had its call for interest in November 2006, and the official standards task force was authorized in May 2007.[2] The IEEE ratified the final standard in September 2010.[3] Some companies introduced technology to reduce the power required for Ethernet before the standard was ratified, using the name Green Ethernet.

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In 2005, all the network interface controllers in the United States (in computers, switches, and routers) used an estimated 5.3 terawatt-hours of electricity.[4] According to a researcher at the Lawrence Berkeley Laboratory, Energy-Efficient Ethernet can potentially save an estimated US$450 million a year in energy costs in the U.S. Most of the savings from homes ($200 million), and offices ($170 million), and the remaining $80 million from data centers.[5]

The power reduction is accomplished in a few ways. In 100 Mbit/s, 1 gigabit and 10 Gbit/s speed data links, energy is used to keep the physical layer transmitters on all the time. If they could be put into "sleep" mode when no data is being sent, that energy could be saved.[5] When the controlling software or firmware decides that no data needs to be sent, it can issue a Low Power Idle (LPI) request to the Ethernet controller physical layer PHY. The PHY will then send LPI symbols for a specified time onto the link, and then disable its transmitter. Refresh signals are sent periodically to maintain link signaling integrity. When there is data to transmit, a normal IDLE signal is sent for a predetermined period of time. The data link is considered to be always operational, as the receive signal circuit remains active even when the transmit path is in sleep mode.[6]

Green Ethernet technology was a superset of the 802.3az standard. In addition to the link load power savings of Energy-Efficient Ethernet, Green Ethernet works in one of two ways. First, it detects link status, allowing each port on the switch to power down into a standby or ‘sleep’ mode when a connected device, such as a computer, is not active. Second, it detects cable length and adjusts the power accordingly. Previous standard switches provide enough power to send a signal up to 100 meters (330 ft).[9] However, this is often unnecessary, especially in the home, where 5 to 10 meters (16 to 33 ft) of cabling are typical between rooms. In addition to the pure power saving benefits of Green Ethernet, backing off the transmit power on shorter cable runs reduces alien crosstalk, and improves the overall performance of the cabling system.

Green Ethernet also encompasses the use of more efficient circuitry in Ethernet chips, and the use of "off-load engines" on Ethernet interface cards intended for network servers.[8]

In April 2008, the term was used for switches, and, in July 2008, introduced into wireless routers.[10] A Wireless LAN scheduler using Wireless N Gigabit routers automatically determines when Wi-Fi radio signals are turned on and off to further reduce energy consumption.

Green Ethernet was first employed on home products. However, low port counts mean that significant cost savings are not going to be made using this technology only in the home. Turning off existing devices when they are idle is likely to provide a more immediate saving.[11] Projected power savings of up to 45 - 80 percent were estimated using Green Ethernet switches,[12] translating into a longer product life due to reduced heat dissipation.[13]